Pencil leads



July 26, 1966 D. L. RIPLEY 3,262,904

PENCIL LEADS Filed Aug. 30, 1961 INVENTOR. Benn/Ls L. R/p/ey IQGENTUnited States Patent 3,262,904 PENCIL LEADS Dennis L. Ripley,Bartlesville, Okla, assignor to The Dow Chemical Company, Midland, Mich,a corporation of Delaware Filed Aug. 30, 1961, Ser. No. 135,067 7Claims. (Cl. 260-41) The present invention relates to an improvedcomposition for and a method of manufacturing Writing cores. Morespecifically, it relates to plastic bonded graphite writing corecompositions and a method of their manufacture for use as pencil leads.

Commercial pencil lead is composed of four principal ingredients:graphite, clay, gums and waxes. In a typical method of preparing leadsaccording to the art, powdered graphite is combined with purified, wetclay and mixed for several days in ball mills or revolving drums. Wateris removed and the mixture is compressed under hydraulic pressurethrough an extrusion die having orifices the size of the desired leads.The extruded lead is cut into lengths of about seven inches and dried atelevated temperatures for several days. The leads are then baked atapproximately 1600 F. for several hours. Losses due to warpage andbreakage plus extensive handling requirements add to the cost of thetime consuming procedures.

It is an object of the present invention to provide an improvedcomposition and method of manufacture for Writing cores which wouldobviate the costly pro cedural and handling problems involved in presentday pencil lead production while yielding a commercially advantageouswriting lead. Other objects of the present invention will becomeapparent from the following specifications and claims.

It has now been discovered that the qualities of present commercialwriting leads are readily reproduced, and in many instances superiorproperties provided, by an improved composition, which comprisesgraphite bonded with organic thermoplastic resinous materials. Thisimproved composition possesses properties which enables an improvedmethod of manufacture to be utilized resulting in greatly increasedproduction economy.

For the purpose of facilitating an understanding of the invention, thereare shown in the drawings certain forms which are to be understood asnot limiting the invention to the precise arrangements andinstrumentalities illustrated.

FIGURE 1 is a representation of a writing. core 10 in accordance withthe invention.

FIGURE 2 is a longitudinal cross sectionalvie-w of a pencil 11comprising a wood case 12 containing a generally centrally disposedgroove 13 having a writing core 10.

FIGURE 3 is a side elevation of a pencil 15 comprising a casing 16 witha tapered end 17 and a pointed end writing core 10. i

The improved composition consists of amorphous, natural flake graphitewhich is finely dispersed in organic thermoplastic resins selected toprovide a writing lead with the desired strength and writing qualities.Leads containing less than 20 percent thermoplastic resin becomeobjectionably weak while those containing more than 45 percentthermoplastic resin tend to become overly waxy. Maximum strength,however, is not obtained with the 45 percent thermoplastic composition.The flexural strength increases as this percentage is decreased from 45percent until a point of maximum strength is reached at a composition ofapproximately 30 percent thermoplastic resins and 70 percent (graphite.

In a preferred embodiment, the thermoplastic resinous material comprisestwo components utilized in a ratio which is varied according to thephysical properties desired in the writing core. One of the componentsemployed is a rigid thermoplastic resin such as styrene polymer or acopolymer of styrene and acrylonitrile having a flexural strength over10,000 psi. The other component is a softer, low melt viscosity resinsuch as branched, high pressure polyethylene.

The improved compositions for writing cores are suitable for use in acontinuous extrusion process utilizing conventional plastic processingtechniques. The lead produced in this manner is ready for immediate use;a valuable property which makes possible continuous process methods ofmanufacture. In this improved method of writing core production theappropriate amounts of thermoplastic resins and graphite are intimatelyblended in conventional plastic processing techniques such as dryblending and mill mixing, extrusion mixing and graphite dispersion in aresin solution followed by evaporation of the volatile component of thedispersion. The mixture is then fabricated into desired shapes byextrusion, molding or similar means.

Flexural strengths of compositions in accordance with the invention weredetermined by placing writing core samples on supports spaced two inchesapart and deflecting the midpoints of these spans at a rate of onehalfinch per minute until the breaking point was reached and the result,expressed as flexural strength in p.s.i., calculated from the followingmodulus of rupture formula:

M C S I where S=the modulus of rupture M =the moment at the break pointC=the distance from the centroid to the outer surface and I==the momentof inertia.

Flexural strength of the writing core is influenced by factors otherthan the composition. Generally, the strength increases with intensivemixing and increased compacting prior to extrusion. The extrusiontemperature also influences the fiexural strength of the writing core.The optimum extrusion temperature determined for various compositions inthe following examples, increased as the proportion of rigidthermoplastic resin component in the mixture was increased. Thistemperature always remained below the decomposition temperatures of thethermoplastic resinous components.

Beneficially, the rigid plastic component used in this invention is apolymer prepared from a vinyl aromatic compound of the formula AR-OR=OHwherein R is selected from the group consisting of hydrogen and methyl,AR is an aromatic group containing up to ten carbon atoms, and the groupCR=OH is attached directly to a carbon atom of the aromatic ring, whichmay have chemically combined in the polymer molecule a minor proportionof such substituents as acrylonitrile.

The following examples are illustrative of the present invention but arenot to be construed as limiting thereof.

Example 1 20 percent styrene-acrylonitrile copolymer (prepared by thepolymerization of about 70 percent styrene and 30 percent acrylonitrile)with a flexural strength of about 17,000 to 19,000 psi. and a meltviscosity of about 14,600 poises when a shearing force of 700,000 dynes/cm. at 227 C. is applied, 10 per-cent branched, high pressurepolyethylene with a melt viscosity of about 375 poises at 450 F. and700,000 dynes/cm. and 70 percent natural flake, amorphous, Mexicangraphite (99 percent passing 325 mesh U.S. sieve size) were dry blendedand densified by milling on rolls at a temperature of 210 C. The mixturewas extruded through a inch screw extruder at an extrusion temperatureof approximately 180 C. The resulting writing core had a flexuralstrength of about 8,000 p.s.i. and a hardness, as determined by multiplecomparisons with commercial writing lead samples bearing numericaldesignations, equivalent to No. 3.

Example 2 A mixture of 25 percent styrene-acrylonitrile copolymer,percent branched, high pressure polyethylene and 70 percent graphite,all as defined in Example 1, was blended, densified and extruded as inExample 1 with the extrusion temperature changed to approximately 225 C.The resulting writing core possessed a flexural strength of about 11,500p.s.i. and a hardness equivalent to No. 4 as determined by multiplecomparisons with commercial samples bearing numerical designations.

Example 3 A mixture of percent styrene-acrylonitrile copolymer, 15percent branched, high pressure polyethylene and 70 percent graphite,all as defined in Example 1, was blended, densified and extruded as inExample 1 with the extrusion temperature changed to approximately 170 C.The resulting writing core possessed a flexural strength of about 7,300p.s.i. and a hardness equivalent to No. 2% determined as in the previousexamples.

Example 4 A mixture of percent styrene-acrylonitrile copolymer, 12 /2percent branched, high pressure polyethylene and 62 /2 percent graphite,all as defined in Example 1, was blended, densified and extruded as inExample 1, with the extrusion temperature changed to approximately 220C. The resulting writing core possessed a flexural strength of about6,800 p.s.i.

Tests of the writing cores described in the preceding examples, both aswood cased pencils and as mechanical pencil leads, demonstratedqualities of writing smoothness, uniformity and visibility of linemarkings, and core flexural strength that in many instances weresuperior to commercially available products. The preparation of writingcores in accordance with the invention is completed in a few hours whiletypical, present day, commercial methods require a week or longer toproduce finished leads.

In general, as the graphite component is increased from 70 percenttowards a maximum of 80 percent the flexural strength of the productwriting core decreases. Reducing the graphite component fromapproximately 70 percent towards a minimum of 55 percent also results inlower flexural strength. In both cases, the flexural strength may beincreased by increasing the ratio of rigid to soft While the foregoingexamples and illustrations describe certain embodiments of the inventionin considerable detail for the purpose of facilitating full and clearunderstanding, it will be understood that many modifications can be madetherein without departing from the scope of the invention.

I claim:

1. A method for making writing cores, in a form ready for use inpencils, which consists essentially in dry blending a mixture of ten tothirty percent of a rigid thermoplastic resin selected from the groupconsisting of polystyrene and styrene-acrylonitrile copolymers, 1 to 20percent of a branched, high pressure polyethylene material and 65 topercent natural flake graphite, compacting the mixture at a temperatureabove the softening point of the rigid thermoplastic resinous material,extruding the so densified mixture through a die with orifices ofappropriate diameter and severing the extrudate into required lengthsfor use in pencils.

2. A composition for writing cores which consists of 65 to 75 percentgraphite, 10 to 30 percent styreneacrylonitrile copolymer and 1 to 20percent branched, high pressure polyethylene.

3. A composition for writing cores consisting essentially of (a) 55 topercent flake graphite and (b) 20 to 45 percent of organic thermoplasticresinous material consisting of (l) predominantly styrene linearthermoplastic polymer with a flexural strength above 10,000 p.s.i. and(2) a branched, high pressure polyethylene with a melt viscosity ofabout 300 to 500 poises at 450 F. under a shearing force of 700,000dynes/cmf 4. A composition for writing cores consisting essentially of(a) 55 to 80 percent flake graphite and (b) 20 to 45 percent of organicthermoplastic resinous material consisting of (1) a branched, highpressure polyethylene with a melt viscosity of about 300 to 500 poisesat 450 F. and 700,000 dynes/cm. and (2) a copolymer of styrene andacrylonitrile with a melt viscosity of about 10,000 to 20,000 poises at227 C. and 700,000 dynes/cm.

5. The method of claim 1 wherein the ratio of rigid thermoplastic resinto polyethylene material is from about 1:1 to 5:1, respectively.

6. A composition for writing cores which consists essentially of from 55to 80 percent graphite, 10 to 30 percent of an organic thermoplasticresin selected from the group consisting of styrene polymers andstyrene-acrylonitrile copolymers, and 0 to 20 percent branched, highpressure polyethylene.

7. A composition for writing cores which consists of 65 to 75 percentgraphite, 10 to 30 percent styrene polymer, and 1 to 20 percentbranched, high pressure polyethylene.

References Cited by the Examiner UNITED STATES PATENTS 2,988,784 6/ 1961Lorenian 83 XR FOREIGN PATENTS 704,063 2/ 1954 Great Britain.

OTHER REFERENCES Smith, Vinyl Resins, Reinhold Plastics ApplicationSeries, Reinhold Pub. Corp., New York, 1958, page 72.

MORRIS LIEBMAN, Primary Examiner.

LAWRENCE CHARLES, Examiner.

R. R. RATH, K. B. CLARKE, I. s. WALDRON,

Assistant Examiners.

1. A MAKING FOR MAKING WRITING CORES, IN A FORM READY FOR USE INPENCILS, WHICH CONSISTS ESSENTIALLY IN DRY BLENDING A MIXTURE OF TEN TOTHIRTY PERCENT OF A RIGID THERMOPLASTIC RESIN SELECTED FROM THE GROUPCONSISTING OF POLYSTYRENE ANS STYRENE-ACRYLONITRILE COPOLYMERS, 1 TO 20PERCENT OF A BRANCHED, HIGH PRESSURE POLYETHYLENE MATERIAL AND 65 TO 75PERCENT NATURAL FLAKE GRAPHITE, COMPACTING THE MIXTURE AT A TEMPERATUREABOVE THE SOFTENING POINT OF THE RIGID THERMOPLASTIC RESINOUS MATERIAL,EXTRUDING THE SO DENSIFIED MIXTURE THROUGH A DIE WITH ORIFICES OFAPPROPRIATE DIAMETER AND SEVERING THE EXTRUDATE INTO REQUIRED LENGTHSFOR USE IN PENCILS.